Some help choosing between two Military "Style" Rucksacks?

Both are essentially medium and large versions of the same bag. Not knowing your intended use or your own physical size and ability makes it hard to tell you which to get. You should be planning to keep the weight of the gear kit you carry between no more than 30% to 40% of your own weight, depending on your fitness and the terrain difficulty. Having a larger pack than you really need will tend to tempt you to carry more than you need which can lead to fatigue, difficulty with balance and reduce your enjoyment of any outing. Unless you are going on an extreme winter expedition and are a large guy (over 1.9 meters and 90 kilos) and are very fit I would recommend the smaller pack of the two. You can always lash extra gear you might occasionally need across the top of the pack or to the front and side accessory straps. I think you will be happier with that choice. Internal frame packs like that are very comfortable to carry if you pack them right. Keep heavy dense items like tents. cookstoves and filled water bottles close to your back inside the compartments and close to your center of gravity (not too high or low). Your sleeping bag should go in the lower hatch — wrap it, even if it has it’s own stuff sack, in a plastic trash bag first so it will not get wet in any downpours (all packs leak somewhat in heavy rain.) Keep small items and clothing in various stuff sacks so it is easy to arrange them and pull them out to find stuff. Pack light bulky items like a down or fleece jacket towards the area of the pack farthest from your body.

I’ve owned and used every style and type of pack in over 40 years of outdoor recreation and teaching backpacking skills. That style is my favorite and is very versatile. It was invented in the 1960’s by the US company Kelty and many companies have adapted it since. Bergen makes a decent product for the money. That smaller one is a good size for multi-day trekking, moderate mountaineering and ski touring.

what will you be doing with them? backpacking? vehicle camping and using these as storage? load carrying? how far will you be traveling with it? what are you planning on carrying with these?
these are the questions you should be asking yourself, then when you have that information, it should either be obvious to you or you can ask on here again with the new information you have, allowing us to give a more informative answer, where this question stands now, we might as well be shooting in the dark, looking for the right answer.